The purpose of this study was to investigate the effect of using co-teaching strategy on ESP students’ vocabulary achievement. Meanwhile, this study sought to know whether co-teaching can affect the three level of vocabulary learning separately (meaning recall, form recall, and meaning recognition). Further, it aimed to examine students’ attitude toward co-teaching instruction. To do so, 40 students from two intact classes (each consists of 20) majoring in law were chosen as the participants of the present study. Oxford Placement Test (OPT), three vocabulary tests (measuring learners’ meaning recall, form recall, and meaning recognition) and an open-ended co-teaching questionnaire were used as data collection instruments. In co-teaching group, students received vocabulary instruction from both general English teacher and their special teacher while in other group single teacher taught vocabulary to the class. The findings of the study generally revealed that those learners who were taught through co-teaching had better performance. Additionally, further analysis of three vocabulary tests revealed the effectiveness of co-teaching on three levels of vocabulary knowledge (form recall, meaning recall, and meaning recognition). Finally, the analysis of co-teaching questionnaire revealed that most of the students preferred co-teaching instruction in their classes. Based on these findings, the most important implication of this study was that the collaboration of special and general English teachers leads to better performance and learning on the part of ESP learners.
